3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) that is an advancement of UMTS (Universal Mobile Telecommunication System) is being introduced with 3GPP release 8.
In 3GPP LTE, OFDMA (orthogonal frequency division multiple access) is used for downlink, and SC-FDMA (single carrier-frequency division multiple access) is used for uplink. To understand OFDMA, OFDM should be known. OFDM may attenuate inter-symbol interference with low complexity and is in use. OFDM converts data serially input into N parallel data pieces and carries the data pieces over N orthogonal sub-carriers. The sub-carriers maintain orthogonality in view of frequency. Meanwhile, OFDMA refers to a multiple access scheme that realizes multiple access by independently providing each user with some of sub-carriers available in the system that adopts OFDM as its modulation scheme.
FIG. 1 illustrates a 3GPP LTE wireless communication system.
As can be seen from FIG. 1, the wireless communication system includes at least one base station (BS) 20. Each base station 20 offers a communication service in a specific geographical area (generally denoted cell) 20a, 20b, and 20c. 
At this time, communication from the base station to a terminal is denoted downlink (DL), and communication from the terminal to the base station is denoted uplink (UL).
Recently, development of 3GPP LTE-A (advanced) that is an evolution of 3GPP LTE is almost done. In LTE-A, carrier aggregation is used.